1. Field of the Invention
The present invention relates to a Raman amplification optical fiber which Raman-amplifies signal light including a plurality of wavelength components when Raman amplification pumping light is supplied thereto, a Raman amplifier including the Raman amplification optical fiber, an optical fiber coil in which the Raman amplification optical fiber is wound in a stacked fashion, and an optical communication system including the Raman amplifier.
2. Related Background Art
Raman amplifiers are those containing a Raman amplification optical fiber as an optical amplifier medium, thus being able to Raman-amplify signal light when Raman amplification pumping light is supplied to the Raman amplification optical fiber, and utilizes Raman shift which is a kind of nonlinear optical phenomenon in the Raman amplification optical fiber. From this viewpoint, it is desirable for the Raman amplification optical fiber to have a high nonlinearity. For example, the Raman amplifier disclosed in Japanese Patent Application Laid-Open No. HEI 11-84440 utilizes a highly nonlinear optical fiber or dispersion-compensating optical fiber as a Raman amplification optical fiber, so as to carry out lumped-constant optical amplification.
The inventors studied conventional Raman amplifiers and, as a result, have found the following problems. Namely, when a highly nonlinear optical fiber is utilized as a Raman amplification optical fiber, the Raman amplification efficiency becomes higher, whereby a large Raman amplification gain can be obtained, or a sufficient Raman amplification gain can be obtained at a relatively short fiber length. However, since the highly nonlinear optical fiber has a high nonlinearity at each signal wavelength (signal channel), the waveform of signal light may deteriorate under the influence of nonlinear optical phenomena such as four-wave mixing.
When a dispersion-compensating optical fiber is utilized as the Raman amplification optical fiber, on the other hand, it not only amplifies the signal light but also can compensate for the chromatic dispersion of the optical fiber transmission line. Also, since the chromatic dispersion of the dispersion-compensating optical fiber has a large absolute value at each signal wavelength, signal waveforms are effectively restrained from deteriorating due to the four-wave mixing in the dispersion-compensating optical fiber. However, since the chromatic dispersion of dispersion-compensating optical fiber has a large absolute value at each signal wavelength, it is necessary that the length of the dispersion-compensating optical fiber be controlled strictly according to the length of the optical fiber transmission line, whereby the degree of freedom in the design of optical fiber transmission lines or the design of Raman amplifiers is small.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide a Raman amplification optical fiber comprising a structure which can Raman-amplify signal light at a high efficiency and effectively restrain signal waveforms from deteriorating due to influences of nonlinear optical phenomena, while improving the degree of freedom in the design of optical fiber transmission lines and Raman amplifiers. It is another object of the present invention to provide a Raman amplifier including the above-mentioned Raman amplification optical fiber, an optical fiber coil in which the Raman amplification optical fiber is wound in a stacked fashion, and an optical communication system including the Raman amplifier.
For overcoming the above-mentioned problems, the Raman amplification optical fiber according to the present invention is an optical device which Raman-amplifies signal light including a plurality of wavelength components when Raman amplification pumping light is supplied thereto, and constitutes a part of an optical fiber transmission line. As characteristics at each wavelength of signal light, the Raman amplification optical fiber has a chromatic dispersion with an absolute value of 6 ps/nm/km or more but 20 ps/nm/km or less, and an effective area Aeff of 20 xcexcm2 or less, preferably less than 15 xcexcm2. More preferably, as a characteristic at each wavelength of signal light, the Raman amplification optical fiber has a Raman gain coefficient GR/Aeff of 0.005 (Wxc2x7m)xe2x88x921 or more.
By setting the effective area or Raman gain coefficient within its corresponding range mentioned above, the Raman amplification optical fiber enables the Raman amplification of signal light at a high efficiency. Also, by setting the absolute value of chromatic dispersion as a characteristic at each wavelength of signal light within the above-mentioned range, the Raman amplification optical fiber can effectively restrain signal waveforms from deteriorating due to influences of nonlinear optical phenomena, thereby improving the degree of freedom in the design of optical fiber transmission lines or Raman amplifiers.
The optical fiber coil according to the present invention includes the above-mentioned Raman amplification optical fiber (the Raman amplification optical fiber according to the present invention) wound in a stacked fashion. When such an optical fiber coil is employed, a lumped-constant Raman amplifier is realized.
The Raman amplifier according to the present invention is an optical device which Raman-amplifies signal light including a plurality of wavelength components introduced from an input end thereof and outputs thus amplified signal light from an output end thereof. The Raman amplifier comprises the above-mentioned Raman amplification optical fiber (the Raman amplifier according to the present invention) and a pumping light supplier. In the Raman amplifier, the Raman amplification optical fiber is disposed between the input and output ends, and constitutes a part of a transmission line between the input and output ends. The pumping light supplier is optically connected to at least one end of the Raman amplification optical fiber, and supplies Raman amplification pumping light to the Raman amplification optical fiber. Preferably, in the Raman amplifier, cumulative chromatic dispersion from the input end to the output end has an absolute value of 100 ps/nm or less at each wavelength of signal light.
Since the above-mentioned Raman amplification optical fiber is employed, the Raman amplifier can yield a high Raman amplification gain and effectively suppress the deterioration in signal waveforms. Also, as a consequence, the Raman amplifier has a high degree of freedom in design. In particular, for suppressing the deterioration in signal waveforms, it will be more favorable if the cumulative chromatic dispersion of the Raman amplification optical fiber has an absolute value of 100 ps/nm or less.
The optical communication system according to the present invention contains the above-mentioned Raman amplifier, and transmits signal light including a plurality of wavelength components, while the Raman amplifier Raman-amplifies the signal light. Preferably, one of the wavelengths included in the signal light in the optical communication system falls within one of C band (1530 nm to 1565 nm), L band (1565 nm to 1625 nm), and S band (1460 nm to 1530 nm). By employing the Raman amplifier, the optical communication system makes it possible to transmit signal light over a long distance, effectively suppresses the deterioration in signal waveforms, and yields a high degree of freedom in design.